Acetic anhydride, a well-known raw material widely used in the chemical industry, is mainly used to produce chemicals such as cellulose acetate and is an important raw material for synthesis of medicines, flavors, dyes, etc. There are currently three industrial processes for producing acetic anhydride, including the ketene process, the acetaldehyde oxidation process and the methyl acetate carbonylation process. Among these, the predominant is the ketene process, which is an old-fashioned and small-scale process and is adopted by many manufacturers; however, due to its high energy consuming and other drawbacks, the largest commercial scale production of acetic anhydride is currently the methyl acetate carbonylation process.
The ketene process is carried out by dissociating one water molecule or methane from the raw material of acetic acid or acetone at a high temperature to form ketene, which then reacts with acetic acid to form acetic anhydride. This process, which must be carried out at a reaction temperature of up to 750° C., will gradually go out of use in the future due to its high energy-consuming demand.
The acetaldehyde oxidation process makes use of the metal catalyst such as Mn, Co, Ni, Cu, etc. to oxidize acetaldehyde into peracetic acid, which further reacts with acetaldehyde to form acetic anhydride and a by-product of water. Acetic anhydride will further be hydrolyzed into acetic acid; that is, the final product will be the mixture of acetic anhydride and acetic acid. Therefore, the yield of acetic anhydride will be decreased.
The methyl acetate carbonylation process for producing acetic anhydride is an expanded application of the methanol carbonylation process for producing acetic acid and makes use of methyl acetate and carbon monoxide as the raw materials to produce acetic anhydride in the presence of transition metal catalysts (such as Rh, Ni, Co and Ir) and iodide promoter. The difference between the methyl acetate carbonylation process for producing acetic anhydride and the methanol carbonylation process for producing acetic acid is the water content of the reaction solution; the reaction solution of the former has to be kept in anhydrous conditions, while the reaction solution of the latter can have 1 to 20 wt. % of water content. Water has a great influence on the stability of the catalyst, and the high water content can facilitate the stability of the catalyst. Therefore, the stability of the catalyst in the anhydrous system of the methyl acetate carbonylation process is a primary problem that should be overcome. To solve the problem, a promoter or a co-catalyst such as alkali metal, phosphonium salt, ammonium salt and transition metal catalysts can be added to promote the stability and activity of the catalytic system. In addition, in the methyl acetate carbonylation process for producing acetic anhydride, a small amount of hydrogen must be added in the carbon monoxide feed gas to maintain the activity of the Rh catalyst.
Adding one or more promoters into the catalytic system to improve and promote the catalytic efficiency of the catalyst is the most important subject in these researches. U.S. Pat. No. 4,002,678 discloses that under an anhydrous condition, a carbonylation reaction is carried out by using nickel and chromium as the catalyst and carbon monoxide and methyl acetate or dimethyl ether as the raw materials in the presence of a halide and a trivalent organo-nitrogen compound or a trivalent organo-phosphorus compound. European Pub. No. 0391680 A1 discloses a process for preparing a carboxylic acid by using an alcohol or its ester under a water-containing condition, in which a quaternary ammonium iodide is used as a stabiliser of the rhodium catalyst. U.S. Pat. No. 4,115,444 discloses a process for preparing acetic anhydride, in which a Group VIIIB noble metal is used as the catalyst, together with multiple promoters comprising at least one metal of Groups IVB, VB, and VIB or a non-noble metal of Group VIIIB, or their compounds and a trivalent organo-nitrogen compound or a trivalent organo-phosphorus compound; the catalyst thereof is rhodium and iridium, the metal promoter is iron, cobalt, nickel, chromium, etc., and the organo-nitrogen compound promoter includes an amine, an imidazole, an imide, an amide, an oxime, etc. China Pub. Nos. 1876239 A and 1778468 A both disclose a catalytic system for synthesis of the carbonyl group of methyl acetate to an acid anhydride, in which a Rh compound is used as the catalyst and different amounts of alkyl iodides, hetero-polyacid salts and alkali metal iodine salts are used as the promoter; the performance of this catalytic system can be improved by the synergistic effect of the hetero-polyacid salts and the catalyst. U.S. Pat. No. 7,553,991 discloses that making use of different nitrogen-containing heterocyclic organic promoters in the carbonylation process to form with the Rh catalyst a stabilized complex can increase the carbonylation reaction rate, and adding such kind of organic promoter can lower the reaction temperature or keep the original reaction rate with a reduced LiI amount, which has the effect of saving energy and reducing production cost. Both U.S. Pat. Nos. 5,298,586 and 4,430,273 clearly disclose that in the Rh-catalyzed carbonylation process for producing carboxylic acid anhydride under anhydrous conditions, adding ionic iodides containing quaternary nitrogen can effectively improve the stability and solubility of the Rh catalyst. Taiwan Patent Application No. 097147075 discloses that adding an ionic liquid containing cations having a nitrogen-containing heterocyclic structure in the carbonylation process can increase the carbonylation reaction rate, and the ionic liquid is easily separated and recovered from the catalytic system due to its features such as thermal stability, chemical stability and low vapor pressure.
In the industrial methyl acetate carbonylation process for synthesizing acetic anhydride, a noble metal and iodide catalytic system is generally used. However, such a system usually produces hardly-soluble tars during the carbonylation process. Cao Yu, et al. of Shanghai Coking & Chemical Corp. had a research on the tar components in carbonylation synthesis for acetic anhydride and investigated the reaction mechanism of tar formation [Shanghai Chemical Industry, 2006, Vol. 31, Issue 07]. It was found that in a continuous carbonylation process, by-products such as acetaldehyde, vinyl acetate, ethylidene diacetate, etc. will be formed at the same time, and these by-products tend to react with the metal ions, especially noble metal ions, in the reactor and form hardly-soluble tars. Because the tar will reduce the activity of the catalyst and even encapsulate the iodides and noble metal catalyst to deactivate the catalyst and terminate the carbonylation reaction, lowering the carbonylation reaction rate, the control of tar formation is one of the key points for improving the carbonylation process.
As mentioned in many prior art patents, the formation of tar residues occurs in the carbonylation process of esters and ethers for producing acetic anhydride and ethylidene diacetate, and the tar residues tend to encapsulate the metal Rh and damage the Rh catalyst. Therefore, current patented technologies all focus on the method of removing the tar residues and recycling and reusing the Rh catalyst. The Rh recycling technologies can roughly classified into the extraction method, the precipitation method, the combustion method and the adsorption separation method. U.S. Pat. Nos. 4,340,569, 4,340,570 and 4,341,741 all disclose the noble metal, including rhodium, can be recycled by pretreatment with an alcohol, concentration via evaporation, treatment with an amine, and then extraction with hydrogen halide. Canada Pat. No. 1171879 discloses extraction of noble metals, including rhodium, with solvents which preferentially dissolve the tars; such solvents include alkanes, cycloalkanes, halogenated alkanes, and aromatic hydrocarbons, and particularly cyclohexane, toluene, and carbon tetrachloride. U.S. Pat. No. 3,920,449 discloses recycling the metal Rh by pyrolysis of the residues. U.S. Pat. No. 3,978,148 discloses recycling the metal Rh by adsorption of the metal Rh on the active carbon. U.S. Pat. No. 3,560,539 discloses making use of hydrogen and hydrides to reduce the formation of hydroxyl group from the carbonyl group in the tar, so as to release and thus recycle the Rh complex.
Therefore, it is still a main challenge in the future to develop a more economic process that can effectively prolong the life of Rh catalyst, reduce the tar formation, lower the complexity of the process and increase the space-time yield of carboxylic acid anhydrides at the same time.